The present invention relates to an aqueous ink composition suitable for use as printing ink and stamping ink in general use, and for use with writing instruments, recording apparatus or the like. More particularly, it relates to an aqueous magenta ink composition for ink-jet printing, which is capable of yielding printed images with high quality (i.e., high clearness and sharpness, etc.), and with excellent resistance to water and light, and which is also capable of providing stable ink ejection, without causing the plugging of the nozzles of the ink-jet printing apparatus, even if it is used continuously for a long period of time or it is used intermittently after a period of non-use of the ink-jet printing apparatus.
Generally, an aqueous ink for ink jet-recording comprises as the main components a dye, a humectant such as a polyhydric alcohol or an ether thereof, and water, and when necessary, additives such as an anti-mold agent.
An aqueous ink for use in ink-jet recording should satisfy the following conditions in order to yield an excellent print over an extended period of time:
First, in order that the ink be compatible with ink droplet formation and control of the direction of the ejected ink droplet streams, it is necessary that its viscosity, surface tension, specific electric conductivity, and density each fall within certain appropriate ranges.
Second, it is necessary that no precipitates be separated, due to coagulation, precipitation, chemical change of slightly soluble components or other causes, from the ink during an extended period of continuous use or storage, or during the periods when the apparatus is not in use. Nor should the physical properties of the ink otherwise change during the above-mentioned periods. If the separated solid components or viscous materials in the ink become affixed around the nozzles, or if the physical properties of the ink change, deviating from the predetermined physical properties achieved at the time of the preparation, the desired recording quality, the desired ink ejection stability, and the desired ink ejection response cannot be obtained.
Third, it is necessary that the ink provide adequately high contrast, clearness and the desired color in the printed image, in particular, for use with full-color printers.
Fourth, it is necessary that the ink for ink-jet recording be such that the printed images are resistant to water, light and physical wear.
Fifth, it is necessary that the images printed by the ink dry rapidly.
In order to satisfy the above first through fourth conditions, it is preferable that the molecular extinction coefficient of the dye employed in the ink composition be high and the solubility of the dye in water and the humectant be also high.
In the conventional magenta aqueous ink compositions, for example, direct dyes such as C.I. Direct Red 1, C.I. Direct Red 11, C.I. Direct Red 37, C.I. Direct Red 62, C.I. Direct Red 75, C.I. Direct Red 87, C.I. Direct Red 89, C.I. Direct Red 95, and C.I. Direct Red 227; and acid dyes such as C.I. Acid Red 1, C.I. Acid Red 8, C.I. Acid Red 87, C.I. Acid Red 94, C.I. Acid Red 115, C.I. Acid Red 131, C.I. Acid Red 144, C.I. Acid Red 152, C.I. Acid Red 154, C.I. Acid Red 186 and C.I. Acid Red 245 are employed.
However, the ink compositions comprising the direct dyes, when used in ink jet-printing, have the shortcomings that (a) the plugging of the nozzles is caused in the course of continuous printing or at the time of intermittent use because of the poor solubility of the direct dyes in water and the humectant, and that (b) the color reproduction is poor. When the aqueous ink compositions comprising the acidic dyes are employed, the durability of the printed images is unsatisfactory because of the poor resistance to light and water.
So far, many proposals have been made for an ink for ink-jet recording, but from the viewpoint of practical use, a satisfactory ink composition which is capable of meeting the above-described requirements has not been found.